1824.] Horizontal and Dipping Needles. 167 



ture of the air, during the day, has a much greater effect upon 

 the intensity of action in the opposing magnets, than I could 

 possibly have imagined. 



On the daily Variation of the Dipping Needle. 



Notwithstanding my observations on the daily change of this 

 instrument have not been so successful as those on the horizon- 

 tal needle, yet it will be proper to say a few words on the sub- 

 ject of the experiments, although I do not intend, in the present 

 instance, to give any numerical results ; those I have obtained 

 not being so uniform as I could wish, nor such as to justify their 

 publication. 



The instrument I employed was made by Messrs. W. and T. 

 Gilbert : it was remarkably free and accurate, and certainly gave 

 results with greater uniformity than any dipping needle I ever 

 used. The needle was only six inches in length, a quarter of an 

 inch broad, and very thin ; it performed in the meridian forty- 

 one vibrations in one hundred seconds, when under the usual 

 terrestrial influence ; and when masked and adjusted by two 

 magnets placed in the line of the dip, it made only fifteen vibra- 

 tions and a half in the same time; the power was therefore 

 reduced about eight times. 



It is not necessary to explain here the means that I employed, 

 and the precautions I took to ensure stability ; it will be suffi- 

 cient to observe, that 1 paid the utmost attention to this essen- 

 tial condition, and that 1 believe my want of success did not 

 arise from any defect in this part of the process, but from the 

 extreme delicacy of this instrument, and the consequent diffi- 

 culty in adjusting it when under the influence of the neutralizing 

 magnets. I tried its action for three weeks in the house, but 

 the jarring of doors and other circumstances prevented me from 

 drawing any conclusions. I then removed it to the garden, to 

 a spot well protected by trees and shrubs, and fixed the entire 

 apparatus to my garden wall, which is exactly in the magnetic 

 meridian ; and further sheltered the whole in the best way I 

 could from the effects of the wind and weather. Indeed the 

 only inconvenience was that I could not leave the needle out in 

 the night, and could therefore only notice what took place in the 

 day time, and this, as I have said above, was not so uniform as I 

 could have desired. 



In general a motion commenced soon after the instrument 

 was adjusted in the morning; but it was not of that gradual and 

 progressive kind which indicated an uniformly increasing or 

 decreasing power, as in the other instrument; it passed, for 

 instance, suddenly from one half or quarter degree, to another 

 more or less, and which sometimes in the course of the day 

 would give a difference in the dip to the amount of a degree ami 

 a half, or even more, but 1 seldom saw in it a tendency to 



